1. Field of the Invention
This invention relates to the conversion of gas oil range petroleum feedstocks into lighter petroleum products by hydrogenation and hydrocracking of such feedstocks in fixed bed catalytic processes.
The present invention also relates to an improvement in such processes whereby substantially all the gas oil range petroleum feedstock fed to the process can be converted to such lighter products.
2. Description of the Prior Art
Fixed bed hydrocracking processes are used in the petroleum industry to convert gas oil range petroleum feedstocks into lighter petroleum products. Such processes perform a function similar to that performed by fluid catalytic cracking processes. The process selected to convert the gas oil range petroleum feedstocks into lighter petroleum products will vary according to a number of factors such as the desired product mix from the process, the type feedstock available for the process and the like. Such considerations are well known to those skilled in the art.
A typical fixed bed catalytic hydrogenation process is shown in FIG. 1 and will be discussed in greater detail hereinafter. In general, such processes comprise charging the gas oil range petroleum feedstock to one or more fixed bed hydrocracking zones containing a hydrocrackin catalyst at hydrocracking conditions to produce a first hydrocracking zone product stream. This product stream is typically charged to a fractionation zone where it is separated into a product stream which is recovered and passed to further processing to produce a variety of products such as gasoline, kerosene, jet fuel, diesel fuel and the like and a bottoms stream which is passed to a second fixed bed hydrocracking zone which contains a hydrocracking catalyst at hydrocracking conditions to produce a second hydrocracking zone product stream. The second hydrocracking zone product stream is then passed to cooling and charged to a fractionating zone which may be the same fractionating zone used to fractionate the product stream from the first hydrocracking zone. The separation of products in the fractionating zone is as discussed previously. It is necessary to cool the product stream from the second hydrocracking zone to a temperature suitable for fractionation and in typical processes of this type quantities of heavy materials accumulate in the bottoms stream from the second hydrocracking zone as a result of the recycling of the bottoms stream from the fractionating zone to the second hydrocracking zone and eventually accumulate in amounts large enough that the heavy materials precipitate in the cooling zone. Previously, such heavy materials have been eliminated by withdrawing a portion of the product stream from the second hydrocracking zone and shipping it to a refinery with a fluid catalytic cracking unit for processing or the like. The portion of the second hydrocracking zone product stream withdrawn is fixed by the amount of heavy material which must be removed to prevent precipitation of the heavy material in the cooling zone. Such remedies for this problem result in a reduction in the amount of lighter, more valuable hydrocarbon products recovered from the process and in considerable expense when the material removed is shipped to another refinery for additional processing. Accordingly, a continuing effort has been directed to the development of a process whereby substantially all the gas oil petroleum feedstocks charged to fixed bed hydrocracking processes can be converted to lighter, more valuable petroleum products in the process.